1. Field of the Invention
The present invention relates to a distance measuring device of a type which projects light beam to an object and receives a reflected light beam. More particularly, it relates to an improvement of the distance measuring device wherein an object located at an infinite distance (the term "infinite distance" used herein means a distance assumed to be an infinite distance in the field of photograph) can be detected as an infinite distance object without failure.
2. Description of the Prior Art
The reliability of the distance measuring device of the above described type is dependent very much on the received amount of reflected light beam and the amount of noise signals caused by the ambient light. When the object is close to the device, the light beam reflected from the object is strong enough so that a light-representing signal produced by a photoelectric cell as a function of the intensity of such a reflected light beam has a high amplitude and is distinguishable from the noise signals caused by the ambient lights. However, as the object becomes distant, the reflected and received light beam becomes weaker, whereby the signal representing the intensity or amount of the reflected and received light beam will not be high enough to distinguish it over the noise signals.
According to the prior art distance measuring devices of the above described type, the signal produced from the photoelectric cell is compared with a predetermined fixed reference voltage which is a little above the noise signal level. It is to be noted that, in the distance measuring device of the above described type, the noise signal becomes great as the ambient light becomes brighter. In the case where the signal obtained from the photoelectric cell is below the reference voltage, it is assumed that the object is located at an infinite distance.
A problem in the prior art distance measuring device is that the reference voltage must be set at a relatively high level such that the signal is distinguished from the noise signals of various levels, not only those obtained under a dark surroundings, but also obtained under a bright surroundings. Otherwise, that is, if the reference voltage does not have a relatively high level, a noise signal will sometimes be above the reference voltage and may be mistaken as a wanted signal. However, if the reference voltage is set at a relatively high level, a signal, which is distinguishable from the noise signals but is below the reference voltage, can not be detected. Therefore, according to the prior art distance measuring device, the object is assumed to be located at an infinite distance from the time when the object is at a relatively close position.
Furthermore, the prior art distance measuring device has such a problem that the supply voltage will be dropped when the LED is actuated to emit the light beam. This problem adversely affects the distance measuring operation.